pgsql: Consider parallel merge joins.

Consider parallel merge joins.

Commit 45be99f8cd5d606086e0a458c9c72910ba8a613d took the position
that performing a merge join in parallel was not likely to work out
well, but this conclusion was greeted with skepticism even at the
time. Whether it was true then or not, it's clearly not true any
more now that we have parallel index scan.

Dilip Kumar, reviewed by Amit Kapila and by me.

Discussion: http://postgr.es/m/CAFiTN-v3=cM6nyFwFGp0fmvY4=kk79Hq9Fgu0u8CSJ-EEq1Tiw@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3bc7dafa9bebbdaa1bbf0da0798d29a8bdaf6a8f

Modified Files
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src/backend/optimizer/path/joinpath.c | 259 ++++++++++++++++++++++++--
src/test/regress/expected/select_parallel.out | 25 +++
src/test/regress/sql/select_parallel.sql | 10 +
3 files changed, 275 insertions(+), 19 deletions(-)